First of all welcome to FDMB! You are in good hands here with a remarkable group of people who have already walked this path and understand how you are feeling. Now take a deep breath because I have some good news to share with you.
I take it from the price your vet quoted that he is talking about the 10ml vial and you don't want that because you will end up with a lot of waste. What you want are the pen refills which are tiny vials that contain 3ml each and come in packages of 5 vials. The insulin is only good for 28 days
IF you leave it unrefrigerated which is what most human users do. If you keep it in the refrigerator, then it will last much longer and usually you can use up each of the small vials. So if you purchase the small vials that will have an expiry date at least two years out, the cost, while initially high, is actually your cost over two years or more (depends on the expiry date and dose of insulin your cat needs.)
Given the price you were quoted, I'm assuming you are likely located in the U.S. Here is a
thread about where other folks here are getting their insulin as many U.S. folks order their insulin from Canada as our prices are much lower.
I'm sure there will be others along shortly to give you lots of good info but I wanted to quickly put your mind at ease about the insulin costs. The most costly part of treatment is the testing strips that are used in a glucometer for testing your cat's glucose level to make sure it's safe to give shots, figure out the best dose, and keep your cat safe. Many vet's do not mention home testing to their clients and some even say it's not necessary, but here we believe it is absolutely necessary to keep our cats safe and healthy. You can use a human glucometer because all of our reference material here is based on use of a human glucometer and we can recommend meters that are the most cost effective too.
Keep the questions coming as the only silly questions are those that don't get asked. All the hands on experience here means we have lots of tips and tricks up our sleeves to make this as inexpensive as possible as well as helping you become comfortable treating your cat.